1. Field of the Invention
The present invention relates to the covulcanization of conjugated diene-containing butyl rubber with halobutyl rubber or butyl rubber.
2. Description of the Prior Art
The expression "butyl rubber" is used in the rubber industry to describe copolymers made from a polymerization reaction mixture having therein from 70 to 99.5% by wt. of an isoolefin which has about 4 to 7 carbon atoms, e.g., isobutylene and about 30 to 0.5% by wt. of a conjugated multiolefin having from about 4 to 14 carbon atoms, e.g., isoprene. The resulting copolymers contain 85 to 99.5% by wt. of combined isoolefin and about 0.5 to 15% of combined multiolefin. The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128 which is incorporated herein by reference.
The polymer backbone of commercial butyl rubber is made up primarily of isobutylene units, with just a few percent of isoprene units. The isoprene units contribute the small amount of unsaturation present in butyl rubber. The basic equation is represented by: ##STR1## which combine in the presence of Friedel-Crafts catalysts to form: ##STR2## where x + z represent the number of isoolefin units incorporated in the butyl rubber, while y represents the number of initial diolefin units present, substantially as randomly inserted units. The conjugated diolefin, isoprene, loses one olefinic linkage upon its essentially random incorporation into the polymer backbone.
Thus, butyl rubber, as presently produced, contains only a small percentage of unsaturation, in the form of the single double bond associated with the isoprene residue which is incorporated more or less randomly throughout the polymer chain.
It has been discovered that butyl rubber could be produced containing conjugated unsaturation, which is essentially randomly distributed along the linear polymer backbone. The general formula may be represented by: ##STR3## where x, y and z have the values previously described, though at least one double bond may lay outside the linear backbone.
This variation may be represented by the formula: ##STR4##
This new butyl rubber has been termed "high reactivity butyl" (HRB) and encompasses the conjugated diene butyl rubber, regardless of where the unsaturation resides in the chain.
The HRB is more completely described in a copending U.S. application Ser. No. 228,727, filed Feb. 23, 1972, U.S. Pat. No. 3,816,371. One of the preferred methods of preparing this butyl rubber is described in copending U.S. application Ser. No. 228,728, U.S. Pat. No. 3,775,387, filed by Francis P. Baldwin, one of the present inventors, on Feb. 23, 1972. Both applications are incorporated herein by reference.
One of the present inventors, Francis P. Baldwin, has described the covulcanization of blends of from 10 to 90 wt % conjugated diene butyl rubber with from 90 to 10 wt. % high unsaturation rubber, such as natural rubber, styrene-butadiene rubber (SBR) and the like, in copending U.S. application Ser. No. 228,727, filed Feb. 23, 1972.
Of the many unusual and interesting features of conjugated diene butyl rubber, it has recently been discovered that the rubber develops its maximum tensile strength at higher carbon black concentrations, when compared with regular butyl or halogenated butyl rubbers. This is particularly true with HAF-LS carbon black, where maximum tensile strength of the conjugated diene butyl occurs with 75-80 parts black per 100 parts rubber (phr), as compared with 50-55 phr black with both butyl and chlorinated butyl rubber. In addition, the conjugated diene butyl rubber cures in about one-fifth the time necessary to cure butyl or halobutyl rubber, even using relative "mild" cure packages.